Typical steam jet ejectors feed high-pressure steam, at relatively high velocity, into the jet ejector. Steam is usually used as the motive fluid because it is readily available; however, an ejector may be designed to work with other gases or vapors as well. For some applications, water and other liquids are sometimes good motive fluids as they condense large quantities of vapor instead of having to compress them. Liquid motive fluids may also compress gases or vapors.
The motive high-pressure steam enters a nozzle and issues into the suction head as a high-velocity, low-pressure jet. The nozzle is an efficient device for converting the enthalpy of high-pressure steam or other fluid into kinetic energy. A suction head connects to the system being evacuated. The high-velocity jet issues from the nozzle and rushes through the suction head.
Gases or vapors from the system being evacuated enter the suction head where they are entrained by the high-velocity motive fluid, which accelerates them to a high velocity and sweeps them into the diffuser. The process in the diffuser is the reverse of that in the nozzle. It transforms a high-velocity, low-pressure jet stream into a high-pressure, low-velocity stream. Thus, in the final stage, the high-velocity stream passes through the diffuser and is exhausted at the pressure of the discharge line.